The exposure of skin to sun light for a long period may cause the characteristic signs of skin photoaging, for example, wrinkles, skin roughness, freckles, etc. and further numerous skin imbalance conditions including skin cancer such as keratosis.
It has been reported that such skin photoaging can be improved by applying the cream preparation containing tretinoin (all-trans-retinoic acid) to skin [see, "Topical Tretinoin Improves Photoaged Skin", JAMA, 259, Vol. 4, pp95, 527-532, Jan, 22/29, 1988, the authors Webb et. al.]. However, since tretinoin is lipid-soluble, it has a low skin absorption and is unstable in the living body and irritable to skin, and further may cause the side effects such as skin xerosis, wound, peeling-off, etc. Due to such side effects tretinoin can be used as the main raw material for pharmaceutical and cosmetic products only with many difficulties. Therefore, it has been urgently required to develop the new functional raw material which still posseses the activity of tretinoin for inhibiting the photoaging and is non-irritable.
As one of the more improved agents for inhibiting skin aging over tretinoin, it has been reported that ester or amide derivatives of 13-/trans-retinoic acid exhibit a therapeutic effect for treating skin disorders and acne. Specifically, it has been reported that 2-(all-trans-retinoyloxy)-4-methoxyacetophenone compound has a relatively low irritability and exhibits an effect of inhibiting skin cancer and photoaging [see, U.S. Pat. No. 4,677,120]; and the compound obtained from esterification of retinoic acid and tetraethylene glycol shows an increase in skin penetrating effect [see, U.S. Pat. No. 4,900,478]. In addition, it has also been reported that N-(4-hydroxy-phenyl)retinamide (4-HPR) and retinoyl .beta.-glucuronide (RAG) maintain the activity of retinoic acid but have a relatively reduced toxicity [see, FASEB J., 10, 1014-1024, 1996].
However, since the above-mentioned esters and amides of retinoic acid are lipid soluble and therefore, naturally have a weak skin absorptivity, it is difficult to much expect an increase in collagen synthesis and an effect of inhibiting elastin decomposition. Further, it has also been indicated that they have the stability problem due to their rapid enzymatic decomposition in the living body. Therefore, although these ester and amide derivatives of retinoic acid may overcome the irritability, which is the typical disadvantage involved in use of the prior retinoid-based raw materials for inhibiting skin aging, to some extent, a desire for the new agent for inhibiting skin aging, which has an increased absorptivity through skin, a maximized cell regeneration ability and an improved stability, still remains.
Thus, the present inventors have extensively studied to resolve the problems involved in the prior retinoid-based skin aging inhibitors and noticed that polyethylene glycol deriatives not only have an excellent surface activating property and an effect of increasing the skin absorptivity due to their hydrophilicity but also have the desirable property of minimizing the residual toxicity because they are dissolved in water so that they can be readily excreted out of the body. According to this, we have developed the novel polyethoxylated retinamide derivative of formula (I) which is obtained by amide condensation of a polyethylene glycol derivative and a retinoic acid derivative. The compound of formula (I) thus developed has a superior chemical stability and skin absorptivity over the prior retinoic acid derivatives and consequently, exhibits an excellent skin regeneration effect.